Truss Design

A “truss” is what you see when you look at a truss bridge from one of its sides. A truss is typically made up of a lot of triangles, but some uncommon truss designs don’t have any. The purpose of a truss is to help a bridge support a load (car, train, person) from any point along the span of the bridge. Without a truss, you simply have a beam bridge.

Overview of Terms

Let’s define a couple terms to help you understand how to study truss design.

Shown here in red is the Truss Frame. The frame is the outermost parts of the truss.

The frame is made up of several parts: Top chord, bottom chord, and two end posts. This diagram shows the frame in an expanded view so you can easily see each part. Practically, you might use different sizes or shapes of wood for each of these parts due to the force being put on each part is different.

Now we will add the truss members, which are shown in black in this diagram. The truss members are simply an arrangement of triangles (most of the time) that transfer the force/s put on the bridge to the ground. The way these triangles are arranged or shaped is the essence of truss design. You will see examples of the most common designs further on this page.

These terms will be helpful to keep in mind as we talk more about truss design. Now let’s take a quick look at the history of truss design, particularly in the United States.

Brief History of Truss Design

While trusses have been used for both roofs and bridges for many centuries, there was an explosion of truss advancement in the 19th century in America. The need for bridges to span longer distances in this era, as well as to hold increasingly heavy loads, brought about many creative solutions in the form of new truss designs.

Three names stand out as true pioneers in these early truss bridges: Timothy Palmer (1751-1821), Louis Wernwag (1770-1843), and Theodore Burr (1771-1822). These men, along with other bridge builders who followed them, designed and built many bridges, especially in New England. Theodore Burr came up with a design that was used in many iconic covered bridges, and some are still standing today. These men came up with practical solutions for bridge building, and did not know or have access to the theory behind their designs.

Interestingly, building bridges in the 18th and early 19th century was more about quality of construction. Skilled carpenters were needed, and most of the engineering was practical and not theoretical. Wood was the primary material available in these early years, but iron and then steel came along and changed everything.

With iron and steel, and the expansion of railroads that carried heavier and heavier loads, new bridge designs were needed. The Howe and Pratt trusses in particular were designed to incorporate iron rods in the truss. These two designs, which you can see from the original patent images, do not look exactly like the truss designs that we associate with those names today. This is a bit of a mystery to me, but you can see semblances of the original designs in the modern depictions. Both the Pratt and Howe patents were very much concerned about methodology of construction more so than the actual design.

Bridge history is fascinating, and there is so much more to learn. This short section is meant to whet your appetite, but now we turn to the application of truss design to model bridge building.

Common trusses used in model bridge building

Each of the following truss designs are very common in both real and model bridges because of their sound engineering and ease of construction. As I mentioned earlier, the key for us model builders is how these designs transfer forces throughout the bridge and eventually to the bridge supports. Each of these designs does that in a different way.

Take some time to read up on each of these designs before deciding on one to use for your bridge. Perhaps you will end up not using any of these designs but creating something on your own based on the principles of force transfer.

Warren Truss

Pratt Truss

Howe Truss

K Truss

I have chosen to highlight these four examples of different trusses to get you started with some very solid examples that you can easily use on your bridge. There are other, more complex, designs that aren’t shown here. You can do a web search for truss design and see many more examples. I’m a fan of keeping things simple, but it is possible that your unique bridge project would benefit from one of the more exotic designs.

If you are interested in learning more about trusses and truss design, check out Truss Fun, Second Edition from Amazon. This is a comprehensive study on the engineering principles behind the design of bridges. It is easy to understand and to follow, and is a great fit for students who are just learning, but advanced enough to be a great resource to those with more experience. For more great resources, see this list of other great bridge books.

How do you determine the optimal width of a model bridge? We have to make an elevated wooden bridge that has a span of 45 cm, a tower height of 7 cm and a maximum height of 15 cm (including the tower). Pls advise. Thanks.

Manuel, are you talking about adding a horizontal line in the middle of the Pratt truss? Don’t do it. Try plugging that design into the Bridge Designer program and see what happens. It is basically adding useless weight.

I am not sure what you mean with the X’s at the beginning and end. Are you talking about lateral bracing?

Im building a bridge out of popsicle sticks and im using the truss design. Im not sure which type of truss is stronger because i will be putting bricks on it to test. I was going to do the Warren truss but the way i built it so far makes the triangle on it not equilateral. I was then thinking about a Howe truss…..What is your opinion?

First Timer, a K truss is a good choice. As long as your construction is done well, it should be a good bridge. However, I am not sure what you mean by your bridge has to be on one plain. Could you explain that for me?

Hey it’s me againthank you very much for the advice i am going to try the welbonde again but it turns out the bridge i am building has to have an opening so “boats” can pass not cars and i was wondering if i put a k-truss on arches would the bridge be strong or is their a different type of method to building those types of bridges
thank you again this website is easy to use for all ages

this website is very usefull even though it did not answer all of my questions on model bridges. With that said this site is still the best one that I know of that has good information on how to make model bridges and the materials and tips to make it a good one!!!

You asked a really good question about the “green” supports. The program shows that they have no load, so why even include them on a bridge?

The reason is that the truss design program is dealing with theory only. However, in a real model bridge scenario, the bridge is going to twist and deform under load. Once the bridge starts to deform, then the forces go wacko. The green members might suddenly go red or blue. By leaving the green members in the design, you are putting a small safety factor into the bridge.

It would be an interesting experiment, however, to see if you can achieve a greater efficiency by leaving the green members out. The bridge might not hold as much, but because it will be lighter it could potentially have a greater efficiency.

are you 11? what do you think would happen if you reversed the k’s? your bridge would only be as strong as your adhesive since all the weight would pull on the supports rather than sit on them… you’d increase the stress on joints considerably!
and what would be the purpose of removing the verticle piece? and those green verticles… sort of necessary to hold up the supports. what’s the point in even having that first support? physics… seriously.